Lifting Machines – Reversibility and Efficiency A machine lifts a load of 1000 N by an effort of 80 N. If the velocity ratio (VR) is 20, classify the machine.

Introduction / Context: In lifting machines, reversibility indicates whether the machine will move backward under the load when effort is removed. Efficiency helps determine this behavior.

Given Data / Assumptions:

• Load W = 1000 N.
• Effort P = 80 N.
• Velocity ratio VR = 20.
• Mechanical advantage MA = W / P.

Concept / Approach: Efficiency η = MA / VR. A practical rule: if η > 50%, the machine is reversible; if η ≤ 50%, it is non-reversible (self-locking). Ideal machines have η = 100%.

Step-by-Step Solution: Compute MA = W / P = 1000 / 80 = 12.5. Compute η = MA / VR = 12.5 / 20 = 0.625 = 62.5%. Since η > 50%, the machine is reversible.

Verification / Alternative check: If the efficiency is this high, frictional losses are not sufficient to prevent back-driving under load when effort is removed.

Why Other Options Are Wrong: Non-reversible: incorrect because η > 50%. Ideal: would require η = 100% (not the case). 'Self-acting only on lowering' and 'Indeterminate': not supported by the data.

Common Pitfalls: Confusing mechanical advantage with velocity ratio; remember η = MA / VR.

Final Answer: Reversible (efficiency > 50%)